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Video Summary

# Subtraction

Quick Navigation Video Lectures Dynamic Practice

Subtraction is really just a specific example of the distributive property that we saw in the previons section. However, it seems to cause students a lot of trouble so we wanted to devote an entire section to it. I'll explain the procedure by working through an example.

 Simplify: $(3x^2 + 2x - 4) - (-3x^2 + x - 2)$ Remember that the minus sign is like having a negative one in front of the expression. We can write that explicitly this way. $(3x^2 + 2x - 4) - 1(-3x^2 + x - 2)$ Now I'll apply the Distributive Property that we saw in the last section to distribute the -1 into the parentheses. $(3x^2 + 2x - 4) + (-1)(-3x^2) + (-1)x + (-1)(-2)$ From here we simplify the problem just like the ones in the last section. First I'll do the arithmetic with the coefficients. $(3x^2 + 2x - 4) + 3x^2 + -x + 2$ Now I'll group the like terms together. $3x^2 + 3x^2 + 2x + -x + 2 - 4$ And finally I'll combine them to get the final answer. $6x^2 + x - 2$

That's the formal way of doing the simplification but it always seems like a lot of work to me. With all those - and + signs floating around, there's just too many opportunities for something to go wrong. I like to think of it this way: To distribute the negative sign, just reverse the signs of every term inside the parentheses. Here's a few more examples.

Here's how it would work with Example 1. Say I pick x = 3. If I substitute this into the original expression, I get:

(2(3)2 + 2) - (3(32) - 3 + 2)
(18 + 2) - (27 - 3 + 2)
20 - 26
-6

Now if I make the same substution into my simplified version, I get:

-(32) + 3 = -9 + 3 = -6

Since I got the same number both times, I'm pretty confident my solution is correct.

# Example 1

Simplify (2x2 + 2) - (3x2 - x + 2).

First, I'm going to distribute the negative sign into the parentheses by reversing the sign of every term inside them.

(2x2 + 2) - 3x2 + x - 2

Notice how the first term became negative. In the original problem, it's a +3x2 so the negative sign made it -3x2. Now I'll combine the like terms:

2x2 + 2 - 3x2 + x - 2 + 2

And finally, I'll combine the like terms together.

(2 - 3)x2 + x - 2 + 2 = -x2 + x

# Example 2

Simplify (x3 + x + 1) - 3(-4x2 + 2).

This problem is similar to the last one in that they both involve subtraction but this one has that additional three in front of the second parentheses. There are a number of ways you can handle this sort of situation. I'm going to do it in two steps: First, I'll distribute the three then I'll deal with the negative sign.

(x3 + x + 1) - 3(-4x2 + 2)
(x3 + x + 1) - (3·(-4x2) + 3·2)
(x3 + x + 1) - (-12x2 + 6)

Notice how the only thing I moved was the 3. The minus sign stayed on the outside. Now I'm going to move the negative sign inside the parentheses by changing the sign of everything inside them.

(x3 + x + 1) + 12x2 - 6

Now I'll group the like terms and combine them to get the final answer.

x3 + 12x2 + x + 1 - 6
x3 + 12x2 + x - 5

# Videos

Dynamic Practice - Subtracting Polynomials